Man is the measure of all things: of things which are, that they are, and of things which are not, that they are not.
"Man is the measure of all things" means that an individual's perception of the world is how the world is, at least for that individual. If a man says that something is true, then it is true; if a man says that something is untrue, then it is untrue. This is the basis of relativism.
I have this in common with the midwives: I am sterile in point of wisdom, and the reproach which has often been brought against me, that I question others but make no reply myself about anything, because I have no wisdom in me, is a true reproach; and the reason of it is this: the god compels me to act as midwife, but has never allowed me to bring forth. I am, then, not at all a wise person myself...
Here, Socrates compares himself to a midwife, stating that he himself is not wise, but instead he helps others give birth to their own wisdom. Socrates tells Theaetetus that his duty is to guide people towards the knowledge they already possess, similarly to how a midwife's duty is to guide a woman through childbirth.
When you have come to a decision in your own mind about something, and declare your opinion to me, this opinion is, according to his doctrine, true to you; let us grant that; but may not the rest of us sit in judgement on your decision, or do we always judge that your opinion is true? Do not myriads of men on each occasion oppose their opinions to yours, believing that your judgement and belief are false?
Here, Socrates asks Theaetetus about problems that arise with Protagoras' definition of "truth" ("Man is the measure of all things"). Protagoras' definition implies that everyone believes that every person's opinion (or "truth") is justified, which Socrates points out is not the case. Even if everyone's truths are relative, that does not mean that people are inclined or obligated to accept someone else's truth. In fact, Socrates says, opinions are judged and opposed by others regularly.
For this feeling of wonder shows that you are a philosopher, since wonder is the only beginning of philosophy.
Socrates tells this to Theaetetus soon after their first meeting. Theaetetus is immediately drawn into Socrates' questioning, which allows the conversation to propel forward. This response, Socrates states, proves that Theaetetus is a philosopher because philosophy begins with curiosity.
